I've been using 6 z levels with a copper spike at the bottom. It has been showing to be mostly non lethal to goblins but still crippling. Though It may actually not be the best design all around.
In a 2 or 3 z-level fall armor actually manages to block the fall damage quite often. The fall is still quite damaging to goblins who come wearing only head and body armor, disabling the limbs which are generally the things you wanna disable anyway. But it may be safer for your own soldiers who are likely fully protected allowing their armor a chance to nullify all the fall damage.
In a previous fort I had some of my own fully armored soldiers fall into my own "Dodge This!" traps, 2 out of 3 times the armor completely nullified all the damage from the fall(ignoring the spike) and of the one that did take damage from the fall the armor nullified 3 of the 'hits' from the fall damage. While goblins who fell in on the other hand, had too many unprotected bits and still ended up crippled and unconscious in the pit roughly 90% of the time.
tl;dr 6 z levels seems to do great, 3 might be safer for your own fully armored and protected soldiers while still being enough to disable less armored enemies.
Actually, they do. Upright spears need to be linked to a lever so they can be raised, and even then it only works if the enemy's already on the tile IIRC, so you can't just have them fall on it.
False, they are built in the upright position, and falling on them triggers them. People believed that they had no effect for basically forever, and nobody ever bothered to actually test it. Anything over a 1 z fall will trigger upright spikes automatically. Been that way since the beginning of the 3D version it seems, despite what was believed.
EDIT: Sorry if that seems a bit much 612, I was always thinking it increased fall damage in some way, but my testing during the early days of 3D was ignored because it was 'common knowledge' that spikes did nothing in falls, and there were no combat logs stating otherwise in those versions for proof. And I tend to get overly bothered by it.